1. Field of the Invention
An object of the invention is a high-voltage generator with a wide dynamic range of power, capable of being used particularly in medicine to supply an X-ray tube. At the same time, it could be used in other fields where high values of power of some hundreds of kilowatts, at very high voltages, of the order of hundreds of kilovolts, are also encountered.
2. Description of the Prior Art
Problems related to the dynamic range of power are encountered essentially when a DC high voltage produced by a generator has to be regulated irrespectively of the power consumed by the load into which this generator delivers power. This can be explained as follows: The high-voltage generators commonly used have a rectifier that converts a low-voltage AC signal given by an electrical distribution network into a low-voltage DC signal. This DC signal is then introduced into a high-frequency inverter, often a resonant inverter, which converts it into a high-frequency AC signal. The voltage of the high-frequency AC signal is then amplified by a voltage step-up transformer. The high-voltage signal is then rectified by a second rectifier to deliver the DC high voltage supply signal of a generator.
In medical applications, this high voltage is used to supply an X-ray tube. The reason for using a high-frequency inverter lies in the amount of space taken up by the capacitors of the second rectifier. For, if this second rectifier is a high-frequency rectifier, the capacitances needed to dampen the ripples of the rectified alternations are all the lower as the frequency of the signal to be rectified is high. In a resonant inverter, electrical current is made to flow in a resonant LC circuit, generally serial, alternating in one direction and then in the other. It is this AC signal that has high voltage and is subsequently rectified. The rectifying of this AC signal, despite a careful choice of the capacitors of the second rectifier, is never perfect. This rectified signal has residual ripples prompted by the electrical consumption in the load: namely, in the X-ray tube. These ripples are proportional to the supply voltage.
When the generator delivers power in a high power mode, all the components are used to the maximum of their ability to perform and ultimately a certain degree of ripple which cannot be got rid of is tolerated. This ripple has the drawback of causing variation in the supply of high voltage to the X-ray tube and, ultimately, of modifying the hardness of the X-radiation emitted by this tube throughout this ripple. This happens, of course, to the detriment of the quality of the radiographic images that can be obtained with such tubes. On going into half-power mode or into a lower power mode, in keeping the same supply voltage and the same component elements, this residual ripple of the high voltage persists, prompting the same effects.
To overcome this drawback, it has already been proposed to use so-called half-bridge inverters instead of so-called full-bridge inverters. In this case, a mid-point is created at the first rectifier of the generator. This mid-point is such that the DC voltage at this mid-point is half of the DC voltage delivered by this first rectifier. A resonant (series) LC circuit is then supplied alternately between this mid-point and each of the two supply terminals of the first rectifier. Under these conditions, the resonant circuits are finally subjected to a supply which is half that in the previous case. The result thereof is that the residual ripples of the high voltage produced are then halved. With a view to reducing costs, it has become customary to use a part of the full-bridge inverters to convert them into half-bridge inverters. In this case, a mid-point of a full-bridge inverter is used, connected to the mid-point of the first rectifier. The connection is made by means of a switch. When this switch is open, the inverter is used as a full bridge. When the switch is closed, the inverter is used as a half bridge.
Such devices, however, have the drawback of not being compatible with one another. For, the switch is an electromechanical switch with a build-up time, when turned on, of about 100 ms. For a medical type of application, this is too long. It will be noted that the ripple frequencies of inverters are generally 10 to 20 KHz. The effect of this incompatibility is that the procedures for using the inverters have to be properly differentiated. In particular, in the invention, it is sought to resolve the problem of reducing the build-up time in a low-power mode by using the fast build-up time characteristics of the full-bridge inverter, during the rising stage of the rectified high voltage. Then, the inverter is switched over to use as a half-bridge inverter as soon as the high voltage is set up because, in the experiments conducted, the operation would be in a low-power mode. The above-mentioned incompatibility among prior connection systems does not allow such switching. The result of this is that, at low power, the generators still have high build-up times whereas they would be capable of faster delivery, for example delivery in one millisecond, of the high voltage expected of them if they were to be supplied as a full bridge. They would have a residual ripple also twice as small if, once this high voltage is reached, it were possible to switch over the inverter of the full-bridge assembly into a half-bridge assembly.
The device of the invention enables precisely this result to be achieved. In particular, it also enables an automatic passage from use as a half bridge to use as a full bridge and vice versa. This passage therefore becomes transparent to the user who no longer has to determine the procedures of use (high power or low power) beforehand. This automatic passage thus enables the generator to work permanently under optimal conditions of the lowest residual ripple given the power required by the load.
The principle of the invention is based on the replacement of the switch which enables the mid-point of the half bridge to be connected to the mid-point of the rectifier by a switch controlled at the high ripple frequency of the inverter. In one particular variant, this switch is even set up by placing two thyristors, in a back to back arrangement, each of these thyristors being triggered alternately, twice in sucession.